This invention relates to improvements in or relating to injection pumps for internal combustion engines of the kind comprising: a pump assembly including a cylinder and a piston reciprocable in said cylinder to deliver liquid fuel from said cylinder during an active portion of a delivery stroke of the piston; structure defining a delivery passage and a return flow passage for fuel between said cylinder and a linking passage for connection to an injector; a normally closed delivery valve positioned in the delivery passage in said structure in alignment with said cylinder and responsive to pressure in the delivery passage to open during the active portion of the delivery stroke of the piston to allow fuel to be delivered to the linking passage; and a normally closed return flow valve positioned in the return flow passage in said structure in alignment with said cylinder and responsive to the exceeding of a given pressure difference between the linking passage and said cylinder to open to allow fuel to flow back from the linking passage towards said cylinder; said return flow valve including a permanently open axial passage forming part of the delivery passage.